Wave and current measurements in the St.Lawrence Estuary near Gros-Cacouna, Quebec : Implication on sediment transport

An exhaustive current meter study was conducted last fall (2007) in the St. Lawrence Estuary in the vicinity of Gros-Cacouna, near Ile Rouge and Ile aux Lievres. Five Acoustic Doppler Current Profilers (ADCP) were deployed to monitor current speed (Cs), current direction (Cd), and wave parameters (significant wave height (Hs) and direction of propagation) over a period of twenty (20) days. Four 600 kHz Sentinel Workhorse were deployed in depths ranging from 25 to 45 m, while one 1200 kHz was deployed in the coastal area in a 15 m depth. The survey period was conducted during both neap and spring tide events. The data was analyzed for near bed, mid-depth, and surface currents at each survey sites according to direction of propagation. In general, the current direction pattern observed at all measurements sites showed that currents were mainly oriented towards North North-East and South South-West. These correspond to the tidal current directions known for this part of the estuary which are parallel to the shoreline. Important differences were observed between ebb and flood tides current speeds. The results show that currents velocity may be described using the following vertical distribution with (1) near surface currents (less than 10 m) with current speeds Cs < 2.5 m/s, (2) intermediate water layer, between 10 and 20 m, characterized by turbulent flow and current speeds Cs equiv 1.5 m/s, and (3) a bottom layer > 20 m where Cs les 1.0 m/s. Significant wave heights measured during the survey period vary between 0.1 and 1.5 m. The highest waves measured (> 1 m) originate from the western direction. The comparison between current measurements and multibeam sounding surveys collected - registered in the same area earlier in fall shows that important hydraulic dunes can be generated, suggesting suspended bed load as a significant sediment transport pattern in some parts of the Estuary.